libstdc++: Document that _GLIBCXX_CONCEPT_CHECKS might be removed in future

The macro-based concept checks are unmaintained and do not support C++11
or later, so reject valid code. If nobody plans to update them we should
consider removing them. Alternatively, we could ignore the macro for
C++11 and later, so they have no effect and don't reject valid code.

libstdc++-v3/ChangeLog:

	* doc/xml/manual/debug.xml: Document that concept checking might
	be removed in future.
	* doc/xml/manual/extensions.xml: Likewise.

libstdc++-v3/doc/xml/manual/debug.xml

@@ -351,6 +351,8 @@
 
   <para> The <link linkend="manual.ext.compile_checks">Compile-Time
   Checks</link> extension has compile-time checks for many algorithms.
+  These checks were designed for C++98 and have not been updated to work
+  with C++11 and later standards. They might be removed at a future date.
   </para>
 </section>
 

libstdc++-v3/doc/xml/manual/extensions.xml

@@ -77,8 +77,7 @@ extensions, be aware of two things:
       object file.  The checks are also cleaner and easier to read and
       understand.
    </para>
-   <para>They are off by default for all versions of GCC from 3.0 to 3.4 (the
-      latest release at the time of writing).
+   <para>They are off by default for all GCC 3.0 and all later versions.
       They can be enabled at configure time with
       <link linkend="manual.intro.setup.configure"><literal>--enable-concept-checks</literal></link>.
       You can enable them on a per-translation-unit basis with
@@ -89,10 +88,17 @@ extensions, be aware of two things:
    </para>
 
    <para>Please note that the concept checks only validate the requirements
-   of the old C++03 standard. C++11 was expected to have first-class
-   support for template parameter constraints based on concepts in the core
-   language. This would have obviated the need for the library-simulated concept
-   checking described above, but was not part of C++11.
+   of the old C++03 standard and reject some valid code that meets the relaxed
+   requirements of C++11 and later standards.
+   C++11 was expected to have first-class support for template parameter
+   constraints based on concepts in the core language.
+   This would have obviated the need for the library-simulated concept checking
+   described above, but was not part of C++11.
+   C++20 adds a different model of concepts, which is now used to constrain
+   some new parts of the C++20 library, e.g. the
+   <filename>&lt;ranges&gt;</filename> header and the new overloads in the
+   <filename>&lt;algorithm&gt;</filename> header for working with ranges.
+   The old library-simulated concept checks might be removed at a future date.
    </para>
 
 </chapter>